Copy lines matching pattern, then substitute

Question

I want to search file for pattern, duplicate every line that matches that pattern, then in perform substitution on each duplicated line.

Suppose file looks like this:

a 1
a 2
b 1
b 2
c 1
c 2

I want to duplicate each line matching /2, then substitute 2 with 3 (s/2/3), and end up with file:

a 1
a 2
a 3
b 1
b 2
b 3
c 1
c 2
c 3

Answer 1

You can do that with a :g command which marks all lines on which to act and then run a combination of a :t (copy) and :s (substitute) command.

Something like this should work:

g/2$/t.|s/2/3

This executes on each line matching 2$ the following command: :t.|s/2/3 which means, copy the current line and afterwards replace the 2 by 3 in the current (copied) line.